In the production of olefins by the thermal cracking of a hydrocarbon feed it is necessary to bring the hydrocarbons in the cracking zone of the cracking furnace to relatively high temperatures, generally about 550.degree. C. to 900.degree. C. if the desired conversion is to be obtained during the brief residence time of the hydrocarbon in this zone.
To make it possible to attain these temperatures within the cracking zone, it has been found to be necessary, or at least desirable, to preheat the hydrocarbon before the feedstock enters the cracking zone, to relatively high temperatures.
Since the cracking generally is carried out in the presence of water vapor (steam), as an inert diluent, it is generally desirable to preheat the steam to the preferred input temperature at the cracking zone.
The high temperatures required in the cracking zone can be attained by passing the feedstock through cracking tubes which extend through a radiation zone of a burner-fired cracking furnace. The hot combustion gases resulting from the use of fuel-fired burners have a relatively high heat content as they leave the cracking zone and can be used for preheating the feed and even other fluids as may be desirable. To this end, the combustion gases before being vented through a stack, are passed through a convection zone of the furnace which is provided with a heat exchanger traversed by the feedstock and/or other fluids to be heated.
Waste heat recovery in this fashion is known in the art. A typical system utilizing heat exchangers in the convection zone is described in Chemical Engineering Progr., Vol. 74, 1978, pp. 45-50.
In this system, the hot combustion gases are initially passed over a heat exchanger in which the feed for the cracking zone is heated to the inlet temperature of this zone. A further heat exchanger, downstream of the first, serves to generate heated high pressure steam while yet another heat exchanger produces process steam which is fed with the hydrocarbons as the feed to the radiation zone at the inlet temperature thereof.
Still other heat exchange processes can be carried out to further cool the combustion gases and abstract heat therefrom before these gases leave the convection zone, e.g. to heat feed water for the steam generating process and/or to raise the hydrocarbon feedstock to a somewhat higher temperature before it is introduced into the first-mentioned heat exchanger.
For the cracking of certain hydrocarbon feedstocks, e.g. in the cracking of ethane or naphtha, the individual heat exchanger in the convection zone must be so dimensioned to allow optimum utilization of the heat content of the combustion gas.
With charging economic conditions, product demand and availability of different feedstocks, however, there is a change in the effectiveness of heat utilization or the heating effect in heat exchangers of given dimensions and this requires changes in the process conditions.
For example, if it is desired to have a constant output of a given cracking product, e.g. ethylene, with variations in the types of feedstock used, one must adjust the quantity of steam used as a diluent, alter the throughput of the feedstock etc.
Such changes in process conditions frequently require sharp changes in the throughputs both of the steam and of the feedstock so that the quantities per unit time traversing the heat exchangers may vary, depending upon the demands placed upon the system, thereby shifting the actual operating conditions of the heat exchanger from the optimum designed condition.
Optimum utilization of the waste heat cannot be ensured and frequently significant amounts of energy are lost or the process deteriorates.